On the compactness of solutions to certain operator inequalities arising from the Likhtarnikov — Yakubovich frequency theorem

We study the compactness property of operator solutions to certain operator inequalities arising from the frequency theorem of Likhtarnikov — Yakubovich for C0-semigroups. We show that the operator solution can be described through solutions of an adjoint problem as it was previously known under some regularity condition. Thus we connect some regularity properties of the semigroup with the compactness of the operator in the general case. We also prove several results useful for checking the non-compactness of operator solutions to Lyapunov inequalities and equations, into which the operator Riccati equation degenerates in certain cases arising in applications. As an example, we apply these theorems for a scalar delay equation posed in a proper Hilbert space and show that the operator solution cannot be compact. This results are related to the author recent work on a non-local reduction principle of cocycles (non-autonomous dynamical systems) in Hilbert spaces.

A low-frequency theorem on laser-assisted X-ray scattering

As we shall show, for laser-assisted X-ray scattering by free electrons, a similar low-frequency theorem can be derived, as was found many years ago by Kroll and Watson, for potential scattering of electrons in a laser field. If quantum mechanical recoil effects are neglected, our renormalized cross-section formula becomes independent of Planck's constant and therefore must have a classical counterpart.